perf top: Access hists->lock only if needed

The perf_top__record_precise_ip() releases and regrabs the
he->hists->lock because it can sleep if there's an error.  But it should
be done conditionally as it slows down the fast path.
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1449802616-16170-2-git-send-email-namhyung@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-top.c

@@ -194,6 +194,7 @@ static void perf_top__record_precise_ip(struct perf_top *top,
 
 	pthread_mutex_unlock(&notes->lock);
 
+	if (unlikely(err)) {
 		/*
 		 * This function is now called with he->hists->lock held.
 		 * Release it before going to sleep.
@@ -209,6 +210,7 @@ static void perf_top__record_precise_ip(struct perf_top *top,
 		}
 
 		pthread_mutex_lock(&he->hists->lock);
+	}
 }
 
 static void perf_top__show_details(struct perf_top *top)